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Have you spoken with Jack at MiniTech? Minitech Machinery
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Hello all, trying to get an older Minitech-2 mill running to it's greatest potential, but having some troubles with precision. For example, trying to cut a 1.000" circular pocket in delrin gives me something closer to 1.008" (despite attempts with different speed/feed/final passes).
I've noticed that I can make the table rock back/forth up/down by up to 0.010" in each direction by pushing with minimal force. I'm guessing this is the source of my problems.
Question 1: should I expect to be able to machine parts accurate to 0.001" with this machine?
and if yes, then Question 2: might this just be a problem of old bearings that need to be replaced? I've attached a picture of the present bearings, that have a plastic lining on the inside (these run on 3/4" rods; I've measured ID of the bearings at 0.754", but maybe the plastic deforms even more under pressure).
finally, Question 3: any suggestions for a good bearing replacement? I've had no luck finding anything similar to these old bearings (a name would be very helpful- "plastic-lined metal-backed sleeve bearings"?). I've also seen a few fellow Minitech-2 refurbishers mention brass bearings; there seem to be several options on McMaster for bronze bearings (oil or graphite filled, etc).
Any hints would be greatly appreciated.
Cheers,
Mike
Have you spoken with Jack at MiniTech? Minitech Machinery
I have one of those exact machines. I call it my RETRO-MILL
I have totally re-worked the machine. Here is what I have done. Maybe it will help you as you want to get every little bit of performance out of it.
First off If you are dealing with the stock electronics and motors, ditch them and go with new generation parts. Will help ALOT
I also replaced the bushing you are asking about. I believe that they are made by Thompson. I did not go this route. I wanted to make my machine a little tighter. Good and bad thing. I found some bearing bronze bushings. Had to machine the OD down so that they would press fit into the blocks. I also had to open the ID up a bit. After measuring the ID I used the lathe to cut a couple .015 thou deep oil passages in the ID.
Then hone them with a brake cylinder hone to get the fit I was after.
Now the machine was way to tight for the tiny acme screws and nut. So....... Ballscrews it was. This was the hardest part. Had to have a friend of mine cut the screw blocks and drill/tap the ballscrew mounting holes in the blocks. For me, turning the ballscrew ends for the motor couplers was easy. Ceramic inserts
After getting all this done, The thing was fast, tight and fun. I was cutting some knee sliders out of delrin for a motorcycle racers leathers when I noticed a little issue. With a 3/8th ball nose endmill in the spindle I noticed that when it made a plunge that the Z column would rock back and up. Flex....... uggggh!!!!! Tracking it down I found that the stainless steel chip pan would bow when resistance was applied to the endmill. Granted I was being very aggressive. So, to solve this problem I got a 1 inch aluminum plate (WAY OVERKILL I KNOW) drilled and countersunk the required holes and put it under the chip plate. Later I took the chip plate out and just left the 1 inch thick plate.
Here is the mill chewing up some plastic. Once it gets to the bottom of the pocket is where it really bites in.
http://www.youtube.com/watch?v=jo7Klxl2PKc&feature=g-upl]older minimill 2 eating plastic - YouTube
Hope this helps
Fix
more pics of the conversion.
A video of the dry run after installing a smooth stepper to the control box.
http://www.youtube.com/watch?v=EzquxcA9OVg&feature=g-upl]Retro mill 2 smooth stepper dry run - YouTube
Question 1: should I expect to be able to machine parts accurate to 0.001" with this machine?
thinking more on your question.
Have you trammed (squared) the mill? If the machine is not square, then it will cut smaller then requested. This is mainly because the spindle sometimes it at an angle. Take a pencil hold it vertical. Now angle to top of the pencil in any direction. Now imagine it going into material trying to cut it. You should notice that not only will the bottom of the pencil will touch the material, but so will one side of the endmill making the cut larger then the diameter of the cutter. If it was straight up and down, the side should not touch.
Now, onto runout. with an endmill in the spindle. Put a dial indicator on the shank of the endmill close to the collet. Rotate by hand the spindle pully on top. If the indicator moves, this is runout. It basically means that your endmill is not spinning true to its center. So depending on the runout of say a .250 in endmill. With runout, it will cause the endmill to run in a larger circle which results in a wider cut.
I do alot of small jewelry wax detail cutting. SOne thing I did to, was to take a NSK spindle bearing pack (very low runout!!!) and belt drive it. Normally NSK spindle setups with brushless motors and VFD`s cost around the $3K range. The lower spindle lower half was $500 I think.
So many things come into play when trying to get the most accuracy possible.
Thanks for the suggestion, Don. I did write to Minitech and got a prompt response from Jack. Nice of him to offer his help on such an old machine. I'll share the gist of his response, in case it's helpful to anyone in the future:
Those bearings were made by Thomson. I had difficulty finding them on Thomson's site, so not sure if they still actively make them. But looks like they are "Nylined" sleeve bearings, Type 1. For my particular machine (3/4 ID, 7/8 OD, 3/4 L), that's part 12N12-D. Found some of these on ebay.
Jack also suggested that these machines were meant more for training at vocational schools and not necessarily the highest accuracy like the newer machines, but with some tweaking I should be able to get the desired accuracy.
Fixittt, thanks for all the helpful information!
I did start with the original electronics/motor/computer/software, and have since upgraded to this century. (Although I can't remember why I thought that was absolutely necessary-- I can see now how updating an old machine can be a slippery slope that ends with a machine that only superficially resembles the original, for a pricetag that approaches a new machine!)
Interesting to hear your route with the bronze bearings. For now, I've ordered inexpensive replacements and will see where that gets me before getting too involved. I now see that the nylon inserts in these bearings easily pop out, so I'm tempted to slide some shim stock between nylon and metal backing to tighten things up as a next step.
The runout seems OK. About 0.001, so that's not causing the 0.008 error that I see. The X and Y axes are not perfectly perpendicular (my squared up blocks aren't quite square), but I also don't think this is the source of the uniform 0.008 error (seems like X/Y being off would cause a slightly oval pocket varying roughly from 1.008 major diameter to 0.992 minor diameter).
I have some ideas for squaring things up and doing a better job of tracking down the source of the error. Once I get the new bearings, I'll give things a try and report back if I learn anything worth passing on. Will also try to find out whether, by "tweaking", Jack means careful squaring/tramming/etc, or if he means more serious mods like your 1 inch aluminum base.
Thanks, and cheers,
Mike
just looked at your pictures again.
Are those the xylotex 269 oz motors?
If so, I will say that I have a couple that have .020-.030 thou worth of end play in them. Meaning that I can push on the shaft of the motor and it moves in and out of the can.
more of an FYI.
As for the shim stock, I wouldnt advise it as now you are creating an uneven contact patch to the shafts.
Yep, Xylotex 269s. Fortunately, I'm not seeing any play on mine (<0.001"). Will double-check when everything is back together. Also minimal backlash with them (though that's without any load). Did yours arrive brand new with that much play, or was it over much use?
Re: shim stock; I meant I would precisely cut a piece of shim stock down to the right dimensions so it would wrap all the way around the inside of the metal backing. I think this would keep the contact uniform (and the slit in the nylon sleeve would readily accommodate the ~0.001" reduced diameter). But hopefully the new bearings will be much tighter and make the shims a moot point.
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